Investigations have shown that the maximum amount of glass which may be drawn from a glass-producing furnace can be increased if the batch materials melted to form the glass (including caustic soda, soda ash, sand, limestone, salt cake and other minor ingredients such as carbon and rouge) are added to the furnace in a granulated or pelletized form instead of the conventional loose batch form. Basically, pelletizing is a process in which finely divided materials are placed on a pelletizing device such as a rotating, inclined disc and sprayed with a liquid for the purpose of forming pellets. The raw materials to be pelletized in the pelletizing device are carried under a liquid spray. Seed pellets are formed when droplets of the sprayed liquid draw a few raw material particles together. In some cases, the process is aided by a compacting action of scrapers acting against the sides and the bottom of the pelletizing device. Seed pellets are carried under the raw material feed stream and are coated with new feed materials. The alternate wetting and coating of the individual pellets plus the rolling and tumbling action imparted to the pellets by the pelletizing device cause the pellets to grow in size. The size to which the pellets grow is controlled by controlling the variables of the pelletizing process.
The ability to form pellets of proper size with an adequate green strength (strength when pellets are wet) does not necessarily mean that when the pellets are dry they will have adequate strength to resist both the crushing and the abrading forces encounted during the handling associated with glass manufacturing operations. It is necessary to thoroughly dry the glass batch pellets prior to placing them in a melting furnace as even a little residual moisture can cause the pellets to explode in the furnace. The dry pellets must be strong enough with withstand the drops, tumbling and storage conditions encountered in a pellet handling and storage system in which the pellets are moved from a pellet dryer to storage and then to a melting furnace.
The prior art teaches that the soda ash of the glass batch materials may be replaced, in whole or in part, by caustic soda. The caustic soda provides the sodium oxide for the final glass which had been previously provided for by the soda ash. This prior art is best illustrated by U.S. Pats. 3,542,534 and 3,726,697 and Canadian Patent 923,209. These patents show the mixing of glass batch materials and the use of caustic soda as a replacement for soda ash. In this prior art, the glass batch materials and the caustic soda are mixed together prior to introduction of the mixture to a pelletizing device. The patents also indicate that it is necessary to reduce the particle size of all the materials involved in the process in order to successfully pelletize.
Our co-pending patent application entitled "A Process for Producing Pre-Treated Glass Batch Materials and the Glass Product Produced Thereby", filed Feb. 27, 1975, and assigned Ser. No. 553,858, describes a process for producing high strength glass batch pellets in which caustic soda is used as a replacement for some or all of the soda ash that would otherwise be used in the glass batch. That application is hereby incorporated by reference. The application teaches that if the average particle size for the dolomite and the limestone used in the glass batch materials is controlled within specific limits, the rest of the materials used in forming the glass batch pellets may be standard, well-known glass making materials. In particular, that application teaches that if the dolomite used has an average (statistical mean) particle size less than 15 microns, preferably less than 7 microns, and the limestone has an average (statistical mean) particle size less than 250 microns, a strong pellet may be produced which resists the crushing and the abrading forces encountered in normal handling operations.
It is an object of this invention to provide a method of manufacturing strong pellets of glass batch materials on a pelletizing disc. It is a further object of this invention to provide a method of making pelletized, glass batch materials in a process using a rotating pelletizing disc in which 70 to 85% of the glass forming materials used are the commercially available materials heretofore employed in the manufacture of glass.